We always wish we could turn back time to fix a mistake or relive old memories. However, currently, this is entirely impossible. So why can’t we reverse time like turning back a clock, or is there some way that we just haven’t discovered yet?
Why Can’t We Turn Back Time?
In a study published in the journal Physical Review Letters, a group of physicists re-examined the definition of the “Arrow of Time” – a concept describing the relentless forward journey of time – and provided a new perspective on how time manifests on universal scales.
For a long time, time has been described by a “past hypothesis”, assuming that any given system starts from a state of low thermodynamic entropy and is subsequently governed by thermodynamics where its entropy increases. In short: the past is low entropy, and the future is high entropy – a concept also known as thermodynamic time asymmetry.
Even if time does not flow, we can still assign it a direction, referred to as an arrow of time. This is an abstract concept that simply means we can define an order of events. An arrow of time points from the past to the future, from earlier events to later events. It is a direction in time in which everything occurs. The important aspect here is to differentiate between a flow of time and a direction of time.
Imagine looking at each frame in a film reel. We can easily define a direction of time based on which frames come first and which come after. We do this despite the fact that we are looking at static images of events with no motion in the frames. Each frame is a snapshot frozen in time.
If this applies on a cosmic scale, it is hypothesized that the Big Bang created the Universe in a state of low thermodynamic entropy, meaning a state of minimal thermodynamic content. Over an infinite period, as the Universe expanded and cooled, the thermodynamic content of this large system increased. Thus, according to this hypothesis, the fundamental time is linked to the level of thermodynamic content, or disorder, in our Universe.
Shortly after the Big Bang, several observational evidences indicated that the environment of the Big Bang was extremely hot with a highly chaotic state of fundamental particles. As the Universe matured and cooled, gravitational forces began to have a greater influence, leading to increasing order and complexity in the Universe – from cooling gas clouds, forming stars, and evolving planets from gravitational collapse. Eventually, elements could combine into organic matter, gradually evolving into life and humans, thus forming the concepts of space and time. Therefore, on a cosmic scale, “chaos” has significantly decreased, rather than increased as the “past hypothesis” assumed.
Scientist Flavio Mercati from the Perimeter Institute for Theoretical Physics in Ontario, Canada, who participated in the investigation, argues that this is a matter of how thermodynamics is measured. Since thermodynamics is a physical quantity that can be represented in different dimensions (similar to energy and temperature), there needs to be an external frame of reference to measure it. “This can be done for subsystems of the universe while the rest of the universe can be established as a reference for them. However, the entire universe – by definition – has nothing outside it to correspond to these measurements,” Mercati wrote in an email responding to the science and technology news outlet Discovery News.
Moreover, physical equations do not even provide a direction in time. Time could flow backward, and the laws of physics would remain unchanged. You might argue that this could simply be a coincidence for physicists. If the direction in which time flows backward is missing from physical equations, then they cannot tell us the whole story. The inability to perceive a direction for time from mathematical equations does not mean that there is no direction of time in the real world.
Scientist Flavio Mercati.
Flavio Mercati provided a slightly complex example by stating that even in the real world, at the atomic level, most processes are reversible in time. If, in a subatomic process, two particles, a and b, collide and then separate, they typically bounce off each other and drift apart again. If you watch a film of such a process and then see it play backward, you would not be able to determine in which direction the process occurred. The process of reversing time still adheres to the laws of physics.
To be more specific, he took an example of two new particles, say c and d, being born and flying away from each other. Again, you could not be certain of the actual order of events if you viewed a film of this process, as the laws of physics state that the reverse process is also possible. Particles c and d could collide to create particles a and b. Therefore, you cannot assign a clear arrow of time to specify the order in which the process occurred.
This contrasts sharply with events happening around us in daily life where we do not need to worry about determining the direction of time. For instance, you never see smoke billowing from a chimney concentrate back into the chimney and get sucked back in. Similarly, you cannot “un-stir” sugar from a cup of coffee once it has dissolved, and you never see a pile of ash in a fire “stop burning” to become a log again.
So what distinguishes these events from subatomic events? How come most of the phenomena we observe around us never happen in reverse? Surely everything ultimately consists of atoms, and at that level, everything is reversible. So at what point in the progression from atoms to smoke rising from a chimney, to coffee cups and logs, does a process become irreversible?
It all comes down to an important law in physics called the second law of thermodynamics. The field of thermodynamics studies heat and its relationship with other forms of energy. Astronomer Arthur Eddington once asserted that the second law of thermodynamics holds a supreme position among all laws of nature. There are three other laws of thermodynamics related to how heat and energy can transform into each other, but none are as significant as the second law.
The second law of thermodynamics states that everything wears down, cools, separates, ages, and decomposes. It explains why sugar dissolves in coffee and never happens in reverse. It also states that an ice cube in a glass will melt because heat always flows from the warmer water to the colder ice and never the other way around.
Complexity is a quantity without a direction that, in its most basic form, describes how a complex system operates. Therefore, if you look at our Universe, complexity is directly linked to time; as time passes, the Universe becomes increasingly structured, which means more ordered. Mercati stated: “The question we sought to answer in our research is: what established these systems in such a low thermodynamic state at the beginning? Our answer is: gravity and its tendency to create order and structure from chaos.”
Typically, the concept of time is described by the “past hypothesis,” which asserts that every system begins at a threshold of extremely low entropy, after which thermodynamic processes occur that increase entropy. In other words, the past is low entropy while the future is high entropy, which is precisely the asymmetrical time hypothesis. In short, entropy is a measure of disorder within a system, quantifying how much everything is disrupted.
To experiment with this idea, Mr. Mercati and his colleagues created basic computer models to simulate particles in a model universe. They discovered that regardless of how the simulation was operated, the complexity of the universe always increased over time and never decreased.
Since the Big Bang, the universe began in a state of minimal complexity (likened to a “hot soup” of chaotic particles and energy). As the universe cooled down, gravity began to take over, causing gases to coalesce, stars to form, and galaxies to develop. The universe became increasingly complex, with gravity acting as the driving force behind this complexity.
As the universe matured, subsystems became sufficiently independent for other forces to create conditions for the arrow of time to manifest in low thermodynamic systems. In these subsystems, much like everyday life on Earth, thermodynamics can take precedence, creating a “thermodynamic arrow of time.”
On a cosmic scale, our perception of time is governed by the continuous growth of complexity, but in these subsystems, thermodynamics dominates. “The universe is a structure with increasing complexity,” Mr. Mercati states, “The universe consists of large galaxies separated by vast distances. In the very distant past, they were closer together. Our conjecture is that our perception of time is the result of an irreversible law of complexity growth.”
The next step of the research will be to seek observational evidence, which Mercati and his research team are currently pursuing. “…we do not know if there will be any support for it, but we know the type of experiment that can test our idea. These are cosmic observations.” At this time, he did not disclose what type of cosmic observations will be studied, but mentioned that they would release information in an exciting upcoming research publication.
Additionally, Professor Jim Al-Khalili of the University of Surrey, author of the extremely popular book “Black Holes, Wormholes and Time Machines,” stated that the so-called “flow of time” is merely an illusion, and the laws of physics say nothing about the flow of time. They tell us how things like atoms, pulleys, levers, clocks, rockets, and stars behave under different forces at specific moments in time, and if we know the state of a system at a certain time, the laws of physics provide us with the rules to calculate its possible states at some future moment.
Professor Jim Al-Khalili.
However, there is no indication of the passage of time contained anywhere. The concept of time flowing, or moving in any way, is entirely absent in physics. We find that, like space, time simply exists; nothing more. Furthermore, genius Albert Einstein also held the view that time is an illusion and even expressed this when trying to comfort the widow of a close friend by saying she should relax her perception that the present moment is any more special than any other moment in the past or future; all moments exist together.
Thus, we can see how difficult it is to understand time when viewing it as a separate concept. Even Jim Al-Khalili humorously remarked that if you attempt to understand the workings of time, you should be prepared to familiarize yourself with Einstein’s theory of special relativity, where he intertwined time with space into a four-dimensional spacetime. Professor Jim Al-Khalili concluded that reversing time is akin to tricking human perception on a quantum level; nevertheless, if executed on a quantum scale, it remains a possibility.
Indeed, in February 2015, Professor Kater Murch from the University of Washington and his colleagues conducted a quantum experiment. They placed a circuit board in a microwave and then fired photons of light at it—where the quantum field of the particle interacts with the circuit board. After the first photons flew through, the analysis results were “hidden,” and experts would predict the results of the next firing together. According to Murch, this is akin to predicting future events, with the highest probability of being correct at only 50%.
Meanwhile, Murch’s group believes that if they knew the state and future development of each particle, the likelihood of accurately predicting the flight results of photons in the experiment could reach up to 90%. He stated that atomic particles are not defined until humans measure them. This means that in the past, the particles were undefined, but when humans conduct research (in the future), they have already taken shape, weight, or full speed. Simply put, actions in the future have changed what occurs in the past.
If this theory is correct, scientists suggest that time and space are symmetrical. In other words, we today only perceive the rapid passage of time, while in reality, time is a two-way “arrow” that can be completely reversed.
